This program project is aimed at understanding the molecular basis of signalling B and T lymphocytes to become activated and differentiate. A multi-disciplinary effort utilizing molecular genetics, cell cloning technology, flow cytometry and protein chemistry will be utilized for the experiments. Thus, a number of T cell-derived lymphokines and their receptors on B lymphocytes are currently being isolated and characterized using monoclonal antibodies, protein chemistry and/or molecular genetics. Using these reagents, the mechanisms underlying activation of B lymphocytes and cytotoxic T lymphocyte precursors are being elucidated. The mechanisms underlying isotype switch in B lymphocytes are being studied by isolating cell populations that preferentially change isotype synthesis from IgM to particular subclasses of IgG. For the study of the generation of cytotoxic T cells (CTL), we will utilize recombinant inbred strains of mice in order to determine the genetic requirements for T help in the induction of CTL. Exon shuffling experiments using MHC products as well as MHC mutants will aid in the determination of structure-function relationships. We will investigate the role of the C2 domain of class I MHC molecules in delivering an activating signal to cytotoxic T lymphocytes (CTL). In order to determine the role of the C2 domain, we will analyze L cells transfected with mouse-human (hybrid) class I genes. We will determine the role of immune response genes in secondary CTL responses in vitro by assaying the ability of (B6 x bm12)F1 animals to respond in vitro to the H-Y alloantigen.